Self-triggering thyratron pulse generator



Patented Feb. 27, 1,951

UNITED STATES P OFFICE SELF-TRIGGERING THYRATRON PULSE GENERATOR Application December 30, 1948, Serial No. 68,266 1 Claim. (01.250 36) The present invention relates to a self-triggering thyratron pulse generator. More specifically, it relates to producing electrical pulses, that may be of extremely short duration, for example, considerably less than a microsecond, by means of a thyratron circuit that requires no separate or external triggering source for its operation.

While it is not broadly new to produce electrical pulses by means of a thyratron, heretofore it has been customary to employ, in addition to the thyratron and its immediate circuits, other separate devices for providing suitable triggering pulses to the said c rcuit, to cause the thyratron to fire at the desired times. Such auxiliary devices usually included electronic tubes and led to complicated apparatus and circuits.

An object of the invention is to provide a circuit, including a thyratron, that is entirely selfcontained and automatic in its operation, for producing short pulses separated by relatively long and uniform intervals of inactivity, which pulses if desired may be of relatively high intensity and very short duration, of the order of one microsecond or less.

Other objects and many of the attendant advantages of this invention will be appreciated readily as the same becomes understood by reference to the following detailed description, when considered in connection with the accompanying drawings, wherein:

Fig. 1 is a diagram of a circuit that may be used in practising the invention; and

Fig. 2 is a graph illustrating the general nature of the pulses produced by the said circuit.

Referring first to the circuit, it will be seen that it is based on a four-element thyratron, that is, a gas-filled thermionic tube 1 of the tetrode type, having the envelope 2, containing a suitable gas, at low pressure, enveloping the cathode 3, control grid 4, shield grid and anode 6. While conventionally a separate battery-energized heater 1 for the cathode 3 is illustrated, it is obvious that alternating current may serve equally well, and also that a directly-heated filamentary cathode may be substituted, if preferred for any reason, although in the latter case isolating inductances may be necessary. It will also be understood that while entirely independent sources I2, 18 and 24 are illustrated, in practice certain of these may be combined into a single source.

The shield grid 5 is maintained at a negative bias of 6 volts, by the conductor ll connected to the negative terminal of a source [2 of electricity,

the positive terminal thereof being grounded by conductor 13. A relatively large-capacitance bypass capacitor !4, shown as 1000 ,u oi, is connected across the source l2. This capacitance, however, is non-critical, provided only that it is sufliciently large to serve as a pulse by-pass.

The anode-6 is connected, through conductors l5 and It and resistor ii, to the positive terminal of a source 58 of electricity, the negative terminal thereof being grounded by conductor I9. This source It, which need not be a battery, suitably may provide about 180 volts, and the anode resistor I! may be of the order of 270,000 ohms.

A timing capacitor 8, which here has a value of f is connected between the control grid 4 and the cathode 3 by means of conductors 9 and I0. In the circuit chosen arbitrarily for illustration, the thyratron is shown as of type 2D21.

Connected to conductors I6 and I0 is a resistor 20, the timing resistor, shown as 5 megohms,

which, with capacitor 8, provides an R. C. circuit. This resistor is thus connected between the control grid 4 and the positive terminal of the source Hi. It will also be seen that the capacitor 3 may charge from the source 18 through resistor 29 and the cathode resistor 2|, the latter being connected between conductor 9 leading to the cathode 3, and the grounded conductor 22.

A storage device, here shown as a delay line 23,01? 1250 ohms characteristic impedance, is connected at one end to the conductor l5 leading to the anode 6. The resistor 2| is likewise made to have a resistance of 1250 ohms. The delay line 23 consists of a continuous helical winding or inductance 25 sheathed within a conductor 26, shown diagrammatically as forming one plate of a capacitor, the other plate of which consists of the said helically-wound inductance 23, the whole being so proportioned that the resulting characteristic impedance is of the order of 1250 ohms. The sheath 26 consists preferably of a shield braid wherein the separate conducting strands are insulated from each other, to prevent it from acting as a closed secondary winding. Storage devices other than the delay line shown may be used, for example, a network having lumped capacitance and inductance, or even a mere capacitor.

It is to be noted that although definite values have been mentioned for the resistances, capacitances and other electrical constants of the entire circuit, these are merely suggestive and would of course apply only to one specific example. Obviously the circuit constants and components may be varied within wide limits, in accordance with the needs of any particular installation.

The operation is as follows:

The present circuit is self-starting, upon application of an anode-supply voltage. However, for simplicity of explanation, let the following conditions be assumed at start: Delay line 23 charged to a potential difference of 180 volts; timing capacitor 8 uncharged; thyratron l nonconducting. Then, upon application of anodesupply voltage the timing capacitor 8 commences charging exponentially through resistor 20 and resistor 2| towards an ultimate value of 180 volts.

Under the specified conditions of anode potential=180 volts, shield grid bias=6 volts, the 2D2l thyratron will fire when its control-grid potential relative to its cathode becomes +43 volts. Therefore, when the potential difference across capacitor 8 reaches this value, the thyratron fires. The further rise of voltage across capacitor 8 is arrested and the latter discharges through the thyratron at an exponential rate due to the flow of current between control-grid and cathode of the said thyratron, now in the ionized state. At the same time, the delay line 23 discharges in the conventional manner through the cathode load resistor 2!, the latter matching the characteristic impedance of the delay line, thus producing the desired output pulse.

The thyratron next becomes non-conducting, since its anode-to-cathode voltage falls below the level required to maintain ionization, as a result of the discharge of the equivalent storage capacitance of the delay line. Following this, both capacitor 8 and the delay line 23 commence charging towards 180 volts. In order to insure that the said line will be fully charged preparatory to the following discharge event, it is necessary that the period between successive pulses be of the order of magnitude of several times the time constant Racs, wherein Ra is the resistance of anode resistor I1 and C represents the equivalent storage capacitance of the delay line, computed or measured at a frequency of the order of magnitude of the pulse-repetition frequency.

In place of the simple R. C. circuit comprising the resistor 28 and capacitor 8, other timing circuits may be provided, making use of various combinations involving resistance, capacitance,

Number and inductance but free from vacuum tubes, and often designated as passive networks.

Attention is called to the fact that the charging current taken by the timing capacitor 8 is of such small magnitude as to cause negligible voltage drop across resistor 2|. Furthermore, an essential advantage of the circuit herein described is that the return lead 9 of capacitor 8 is connected directly to the cathode of the thyratron instead of to ground. As a result of this, the discharge current of said capacitor, which is much larger than its charging current, does not pass through resistor 2|, and thereby distort the output waveform.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claim the invention may be practiced otherwise than as specifically described.

What is claimed is:

A pulse generating circuit including a gaseous tetrode, having a cathode, a control grid, a shield grid and an anode, a source of electricity for energizing said anode, a resistor connecting the positive terminal of said source to said anode, a timing capacitor having its terminals connected to the cathode and the control grid respectively, a timing resistor connecting the said control grid to the positive terminal of the source of electricity, means for negatively biasing the shield grid, a delay line comprising a continuous winding connected at one end to the anode, and a grounded sheath surrounding said winding, and a resistor of substantially the same equivalent impedance as said delay line, and connecting the cathode and ground.

THOMAS F. C. MUCHMORE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date 2,172,050 Mayberry Sept. 5, 1939 2,212,173 Wheeler et a1 Aug. 20, 1940 2,240,788 Kock May 6, 1941 2 375,575 Morland May 8, 1945 2,390,659 Morrison Dec. 11, 1945 

